1-2 SQUARE MIL The square mil is a unit of measurement used to determine the cross-sectional area of a square or rectangular conductor (views A and B of figure 1-1). A square mil is defined as the area of a square, the sides of which are each 1 mil. To obtain the cross-sectional area of a square conductor, multiply the dimension of any side of the square by itself. For example, assume that you have a square conductor with a side dimension of 3 mils. Multiply 3 mils by itself (3 mils 3 mils). This gives you a cross-sectional area of 9 square mils. Figure 1-1.—Cross-sectional areas of conductors. Q1. State the reason for the establishment of a "unit size" for conductors. Q2. Calculate the diameter in MILS of a conductor that has a diameter of 0.375 inch. Q3. Define a mil-foot. To determine the cross-sectional area of a rectangular conductor, multiply the length times the width of the end face of the conductor (side is expressed in mils). For example, assume that one side of the rectangular cross-sectional area is 6 mils and the other side is 3 mils. Multiply 6 mils 3 mils, which equals 18 square mils. Here is another example. Assume that a conductor is 3/8 inch thick and 4 inches wide. The 3/8 inch can be expressed in decimal form as 0.375 inch. Since 1 mil equals 0.001 inch, the thickness of the conductor will be 0.001 0.375, or 375 mils. Since the width is 4 inches and there are 1,000 mils per inch, the width will be 4 1,000, or 4,000 mils. To determine the cross-sectional area, multiply the length by the width; or 375 mils 4,000 mils. The area will be 1,500,000 square mils. Q4. Define a square mil as it relates to a square conductor. CIRCULAR MIL The circular mil is the standard unit of measurement of a round wire cross-sectional area (view C of figure 1-1). This unit of measurement is found in American and English wire tables. The diameter of a round conductor (wire) used to conduct electricity may be only a fraction of an inch. Therefore, it is convenient to express this diameter in mils to avoid using decimals. For example, the diameter of a wire is expressed as 25 mils instead of 0.025 inch. A circular mil is the area of a circle having a diameter of 1 mil, as shown in view B of figure 1-2. The area in circular mils of a round conductor is obtained by squaring the diameter, measured in mils. Thus, a wire having a diameter of 25 mils has an area of 25^{2}, or 625 circular mils. To determine the number of square mils in the same conductor, apply the conventional formula for determining the area of a circle (A = pr^{2}). In this formula, A (area) is the unknown and is equal to the cross-sectional area in square mils, pis the constant 3.14, and r is the radius of the circle, or half the diameter (D). Through substitution, A = 3.14, and (12.5)^{2}; therefore, 3.14 156.25 = 490.625

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